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Sjoerd
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Wing Flex - Less Flex: Pros And Cons?

Mon Oct 26, 2015 11:07 am

The difference between the B787 wing flex (very flexible) and the A350 wing flex (rigid it seems) is striking. At least from the prictures that circulate so far. Anybody can share further insights as to what the pros and cons are of a lot of wing flexibility and of little wing flexibility?
 
tortugamon
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RE: Wing Flex - Less Flex: Pros And Cons?

Mon Oct 26, 2015 11:28 am

This may help:

Boeing Vs. Airbus Wing Design Philosophies (by ferpe Apr 16 2012 in Tech Ops)

tortugamon
 
celestar
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RE: Wing Flex - Less Flex: Pros And Cons?

Tue Oct 27, 2015 1:37 am

I am always interested in the differences between Airbus and Boeing in designing their plane.
When flying the 787, that wing flex is exciting to watch, scary to think about the stress.
Love to stay tune on this thread to see some good answer to your post.
 
trnswrld
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RE: Wing Flex - Less Flex: Pros And Cons?

Tue Oct 27, 2015 5:40 pm

Another thing I have noticed since early in large commercial aircraft is that it seems older aircraft have much more rigid wings. For example L1011, DC10, 747-1/2/3, A300/310 to name a few show very little if any wing flex. Obviously they bounce around in turbulence, but the kind of wing flex we see in a 787 or a 747-400 during takeoff is something that you definitely never see in any of the earlier wide bodies. I always wondered why this is. In pictures or videos of the above mentioned aircraft the wings remain straight as can be even in very heavy takeoffs. I would say the 767 was the first wide body that started to show some flex.
So what changed in the design and construction of aircraft where wing flex is much more prominent? Maybe older aircraft were overbuilt and maybe it just comes down to them being so strong and stiff and the use the different materials made them very rigid?
 
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Matt6461
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RE: Wing Flex - Less Flex: Pros And Cons?

Tue Oct 27, 2015 8:19 pm

The difference may be unique to 787-9/-8 versus A350-900. The A359 has significantly lower wing loading. The A35K will increase wing loading, and therefore will probably use more of the built-in margin for flex than do the current 787's.

In general, wing flex/dihedral decreases the effective wing area and/or effective lift. It also likely has some implications for effective span, though I haven't read anything specific about this factor.

The impact on effective span would likely be mitigated by flex's winglet-like effect on wingtip vortices.

EDIT- It's also relevant that the 787 has raked wingtips, while the A350 has winglets. Given that wing flex has some winglet-like effect on tip vortices, and that the A350 has actual winglets, I'd expect the optimal flex solution to move further towards flex for the 787 than for the A350.

[Edited 2015-10-27 13:25:36]

[Edited 2015-10-27 14:01:58]
 
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DocLightning
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RE: Wing Flex - Less Flex: Pros And Cons?

Wed Oct 28, 2015 12:02 am

Quoting Matt6461 (Reply 4):
The impact on effective span would likely be mitigated by flex's winglet-like effect on wingtip vortices.

While it is true that increasing upward deflection would somewhat reduce the magnitude of sidewash at the wingtips, the wingtip extensions on the 787 have the most effect towards reducing overall induced drag by increasing span.

Flex is a cost/benefit calculation. Less flex requires a more rigid (heavier) wing structure. It also means that more gust load will be transmitted to the fuselage rather than being damped out by the wing structure.

But less flex also means more effective wingspan per true wingspan.

There are a number of other factors that go into the decision about wing rigidity, as you'll see in the link in Reply 1.
 
Max Q
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RE: Wing Flex - Less Flex: Pros And Cons?

Wed Oct 28, 2015 9:38 am

Quoting trnswrld (Reply 3):
Another thing I have noticed since early in large commercial aircraft is that it seems older aircraft have much more rigid wings.

You should look out the window of a 707 or 727 in turbulence before you say that, they flexed a LOT, and gave a great ride as a result as did the 747 Classic.
 
trnswrld
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RE: Wing Flex - Less Flex: Pros And Cons?

Wed Oct 28, 2015 1:19 pm

Quoting Max Q (Reply 6):
You should look out the window of a 707 or 727 in turbulence before you say that, they flexed a LOT, and gave a great ride as a result as did the 747 Classic

I completely understand that and yes the wings bounce all over the place in turbulence, but I'm more so speaking of the flex you would see during departure in a 744, 787, and A330 for example. The "old" planes never show even the slightest hint of flex. You are obviously right though, in turbulence sitting near the wing on a 727 with all those flaps extended is a site to see.
 
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DocLightning
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RE: Wing Flex - Less Flex: Pros And Cons?

Wed Oct 28, 2015 9:26 pm

Quoting trnswrld (Reply 3):
So what changed in the design and construction of aircraft where wing flex is much more prominent? Maybe older aircraft were overbuilt and maybe it just comes down to them being so strong and stiff and the use the different materials made them very rigid?

The DC-10 and L-1011 had much shorter wings than modern widebodies like the A330 and 777. That said, even the 747 had a fair amount of wing flex visible from the cabin, especially the 744 with the winglets shifting the center of lift more outboard. The 744 was THE classic "flexy-wing" airliner.

The longer wings with their higher aspect ratio are much more aerodynamically efficient but they require a structure that can withstand the higher bending moment of a more outboard center of lift. That typically means more visible flex.
 
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flylku
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RE: Wing Flex - Less Flex: Pros And Cons?

Wed Oct 28, 2015 11:15 pm

I am always amazed that they can engineer the moving parts on the wing to work throughout the flex envelope.
 
Aircellist
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RE: Wing Flex - Less Flex: Pros And Cons?

Thu Oct 29, 2015 1:56 am

Quoting tortugamon (Reply 1):
This may help:

That was one of the best threads in the history of A.net.
 
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ATA L1011
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RE: Wing Flex - Less Flex: Pros And Cons?

Thu Oct 29, 2015 2:18 am

The Tristar and Classic 747's showed some flex on heavy departures, the DC-10 not as much as it wings were angled slightly downward instead of upwards so it did not show as much. It did show slight flex although on heavy departures as did the MD-11. As trnswrld said it really became more apparent when the 767 hit the scene, 767's showed lots of flex especially on heavy departures. Every widebody since has a lot of flex on takeoffs, a lot of it has to do with the more flexible alloys used versus the older days as well. One thing though, Tristars, 747 Classics, DC-10's despite there wings being more rigid than current aircraft they handled turbulence much better than some of these current widebodies, they remind me of like riding in a 75 Coupe Deville or 77 Thunderbird vs a 2010 Charger. You really did not feel the bumps like you would in the Charger as they sorta floated over them!
 
dynamicsguy
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RE: Wing Flex - Less Flex: Pros And Cons?

Thu Oct 29, 2015 10:33 am

Quoting flylku (Reply 9):
I am always amazed that they can engineer the moving parts on the wing to work throughout the flex envelope.

You just have to be careful with how you constrain the degrees of freedom to allow float and/or rotation were needed.

The really hard part is getting the gaps to the spoilers just right while taking into account wing flex and deformation under aero loads of all of the parts.
 
mpgunner
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RE: Wing Flex - Less Flex: Pros And Cons?

Sun Nov 01, 2015 10:27 pm

Just went on my first 787 (HNL to SYD) and was impressed with how smooth the flight was. But I also wondered if the wing flex on the 787 did help with smoothing out turbulence. We had some bumps but they were pretty small.
 
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DocLightning
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RE: Wing Flex - Less Flex: Pros And Cons?

Sun Nov 01, 2015 10:53 pm

Quoting mpgunner (Reply 13):

Just went on my first 787 (HNL to SYD) and was impressed with how smooth the flight was. But I also wondered if the wing flex on the 787 did help with smoothing out turbulence. We had some bumps but they were pretty small.

Probably helps a lot. More flexion to damp out the forces. On my second 787 flight we flew through some turbulence and I saw the wings bouncing all over the place but the ride was still smoother than I would have expected for an aircraft of that size. Now, obviously as a passenger I have no idea about the sorts of forces that the aircraft is feeling and the wing bounce is probably exaggerated in the 787 so I don't know how a 767 would have felt in the same air.

Also, the 787 has a very responsive pitch and yaw control in autopilot very good at damping out a lot of the more annoying movements in airliners.
 
WIederling
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RE: Wing Flex - Less Flex: Pros And Cons?

Sun Nov 01, 2015 11:05 pm

Quoting DocLightning (Reply 5):
Flex is a cost/benefit calculation. Less flex requires a more rigid (heavier) wing structure. It also means that more gust load will be transmitted to the fuselage rather than being damped out by the wing structure.

That is not a given.

for looking into bending a wing is composed of two loadbearing skins kept apart by the wing spars.
for the same bending moment higher wing spars require less load bearing material in the skins.
This scales imu with inverse height. double the height, half the skin loading.

A higher wing also is stiffer, second moment of inertia is a cube relation.
This is reduced to a square relation via the reduction in required skin material.
This should thusly result in a lighter wing for the same material limits.

Airbus does thicker profiles than Boeing. ( compare images presenting sideviews
of the fuselage/wingbox to wing interface of both types )
Still more wing height ( thicker profile ) will reduce flex in a square relationship.

finally: while Boeing seems to do unloaded straight wings
Airbus tends to do unloaded gulled/hanging wings ( rather extremely visible on the A380.)
A loaded B Wing is bent upwards from a straight start.
A loaded A Wing is bent to about straight from a bent down unloaded state.

Thus those two effects bring less visible flex to Airbus wings. IMU and all that jazz.
( one is hard physics the other is a bit of the betrayed eye effect.)
 
nomadd22
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RE: Wing Flex - Less Flex: Pros And Cons?

Mon Nov 02, 2015 2:35 am

Quoting Aircellist (Reply 10):
That was one of the best threads in the history of A.net.

No kidding. Ferpe was magnificent. An example of what this forum could be.

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